This invention relates to a method and apparatus for treating articles with particulate material and to articles treated by the method. The invention may be employed for thermally toughening glass sheets and glass lenses.
It has already been proposed to effect thermal treatment of articles, for example glass sheets, by contacting the articles with particulate material in a fluidised state. In one such proposal a glass sheet is thermally toughened by heating the sheet to a temperature above its strain point and then quenching the sheet in a fluidised bed of particulate material, for example alumina.
It has been preferred to maintain the particulate material in a quiescent uniformly expanded state of particulate fluidisation. To achieve such a state of fluidisation the density of the particulate material must not be so great that the material can only be fluidised in a bubbling state.
The intensity of the thermal treatment of articles, in particular the rate of extraction of heat from the surfaces of a glass sheet which is being thermally toughened by being quenched in the fluidised particulate material, is increased if the thermal capacity of the material can be increased. This would suggest the use of denser, and therefore heavier, particles, and there is a problem in producing a non-bubbling dispersion of such heavier particles which treats the surfaces of the article uniformly and has a high heat transfer coefficient with respect to the surfaces of the article being treated.
In particular there is a problem in maintaining in a non-bubbling state a bed of high density particulate material of sufficient depth for quenching large glass sheets which are used singly as a vehicle windscreen or as a component of a laminated windscreen.
There have been proposals for suppressing bubbling in fluidised beds of inert materials such as catalyst materials.
In U.S. Pat. No. 3,439,899 there is disclosed a proposal for fluidising a mass of particles which are not fluidisable by gas, for example sand, by mixing the sand with permanent magnet particles, for example barium ferrite permanent magnet particles, passing gas upwardly through the mass, and subjecting the particles to a magnetic field which varies in intensity and direction to impart sufficient individual motion to the permanent magnet particles to enable the particulate mass to become fluidised. A small fluidised bed up to about 25 cm deep could be produced in this way.
United Kingdom Specification No. 1,525,754 discloses a magnetically stabilised fluidised bed in which the formation of bubbles is suppressed by mixing with the silica, alumina or catalyst particles of the bed a proportion of magnetizable material, and subjecting the bed to a uniform magnetic field acting in a direction opposite to the fluidising gas. All ferromagnetic and ferrimagnetic substances may be used as the magnetizable material including ferrites of the form XO.Fe.sub.2 O.sub.3 wherein X is a metal or a mixture of metals such as zinc, manganese or copper.
United Kingdom Specification No. 2,002,254 discloses fluidised catalytic cracking catalysts, in particular the zeolite type catalysts in which a crystalline aluminosilicate is dispersed with a siliceous matrix. Bubbling is suppressed by mixing with the catalyst material a magnetizable substance such as a powdered ferrite of the form XO.Fe.sub.2 O.sub.3, wherein X is a metal or a mixture of metals such as manganese, copper, barium and strontium. The mixture is subjected to a magnetizing field to magnetize the magnetizable particles which then exert magnetic attractive forces on one another which stabilize the fluidised bed.